The present invention relates generally to cold cathode devices and associated applications and, more particularly, to a closed-loop circuit that can stabilize the emission of the cold cathode.
A cold cathode is an electron emitter whose emission mechanism is field-based rather than temperature-based. Field-based electron emission occurs when a high electric field is applied to the surface of the cathode""s emitter material residing in a vacuum environment. A tunneling effect allows electrons to pass from the emitter into the vacuum, producing a flow of electrons as defined by the Fowler-Nordheim equation. This tunneling effect is strongly dependent on the surface work function of the emitter material. The surface work function is an inherent property of the emitter material that is affected by surface contamination. Since no vacuum environment can be contamination-free, there is a continuous flux of surface contaminants being carried onto and off of the emitter surface, resulting in wide fluctuations of the emission current on both short and long time scales.
The surface contamination effect is especially problematic in cold cathode applications. Unlike thermal cathodes, which operate at temperatures above 1 000xc2x0 C., the cold cathode is incapable of boiling off impurities on the surface of the emitter. In devices utilizing cold cathodes water vapor and other background gasses are constantly being absorbed and desorbed on the surface of the cathode which changes the surface work function and causes fluctuations in the emission. Thermionic cathodes, which are commonly used in many applications, such as CRT displays, vacuum tubes for audio amplification, vacuum tubes for microwave power amplification, and cathodes for analytical instrumentation and the like, characteristically differ from cold cathodes in that they employ temperature-based electron emission. The electron emission from thermionic cathodes is a function of the power applied to the cathode that determines the operating temperature of the cathode through Joule heating. Thus, the thermionic cathode is considered a power-controlled current source. When the cathode is emitting the maximum current possible for a given temperature, it is said to be operating in emission-limited mode. This mode produces the highest operating current for a given temperature, but can produce unstable emission since the emission from the thermionic cathode is also a strong function of the surface work function. However, since these cathodes are typically operated at above 100020  C. adsorption of contaminants is minimized, so thermionic cathodes typically operate at much lower noise levels than cold cathodes and thus stabilization concerns are not as prevalent. Further stabilization of the emission current of a thermionic cathode can be achieved by operating the cathode in space-charge-limited mode, where the anode voltage and geometry limit the maximum allowed current density as defined by the Langmuir-Child relation.
Since the emission from cold cathodes is field-based, and fields are generally a function of applied voltage, cold cathodes are considered voltage-controlled current sources. When the cathode is operating at the maximum emission current possible for a given control voltage, the cold cathode is said to be operating in emission-limited mode. This mode maximizes the output current from the cathode, but, as described above, produces unstable or noisy emission current. Stability of the cold cathode is a paramount concern in most electron-beam devices that employ a cold cathode, such as audio, video or radar applications. In radar applications noise translates into a degraded signal that reduces the overall detection efficiency. In video applications, noise translates into visual artifacts or uneven brightness present on the monitor.
Several schemes to minimize emission current noise in cold cathodes have recently been developed. See for example, U.S. Pat. Nos. 5,847,408, entitled xe2x80x9cField Emission Devicexe2x80x9d, issued Dec. 8, 1998 in the name of inventors Kanemaru, et. al. and 5,173,634, entitled xe2x80x9cCurrent Regulated Field-Emission Devicexe2x80x9d, issued Dec. 22, 1192 in the name of inventor Kane. Of these schemes, the most prevalent are passive stabilization, most often embodied by incorporation of a resistor or resistive layer into the cathode, and active stabilization, most often embodied by incorporation of a MOSFET or other type of transistor in the cathode circuit. Both of these schemes are open-loop control schemes; that is, they attempt to control emission current without incorporating any measurement of the actual emission current for corrective feedback. Additionally, material changes to the cathode, in the form of changes to the emitter material and the use of coatings on the emitter, have been proposed as means of limiting noise and increasing cathode stabilization.
Passive stabilization seeks to control the emission current by reducing the applied voltage as the emitted current increases. This open-loop scheme is only partially successful, because the emitted current tends to be an exponential function of the applied voltage, while the resistor can provide only a linear reduction in voltage as a function of the emission current. In other words, it is mathematically impossible for the linear passive stabilization scheme to keep up with the exponential fluctuations in emission.
Active stabilization represents an improvement over passive stabilization, in that the transistor element is used as a current-limiting element in the circuit. In this open-loop scheme, the transistor limits the supply of electrons to the cathode, which in turn limits the emission current. In other words, the cathode can only emit electrons if electrons are available. Under these conditions, the cathode is said to be operating in supply-limited mode. This operating mode for cold cathodes is analogous to space-charge-limited mode for thermionic cathodes. In this mode, the stability of the emitted current is directly dependent on the stability of the current-limiting element, which can vary greatly depending on external factors such as temperature, supply voltage variations, and others. Thus, in active stabilization schemes if the current-limiting element is unstable, the resulting emitted current will also be unstable. This is apparent because of the open-loop circuitry which provides for no measurement of the actual emission current for corrective feedback.
Additionally, attempts have been made to address cold cathode stabilization problems by using different emitter materials or coatings on the cathode. Coatings have been used to make the surface of the emitter more inert and thereby raise the surface work function. Alternately, cathodes have been fabricated out of highly resistive materials to allow for a negative feedback mechanism to be built into the cathode""s structure. Implementation of these material changes to the cold cathode has proven to have minimal positive effect on the stabilization concerns.
A desired cold cathode stabilization scheme would depart from these open-loop circuit methods in that the stabilization scheme would control emission current by providing corrective feedback to a measured emission current. It would be desirable to provide for a cold cathode circuit that regulates the emission current using closed-loop feedback. Additionally, the emission current of the cold cathode would benefit from regulation that can be provided remotely, without direct adjustment of the circuit.
The present invention provides for improved stability for the emission current of cold cathode technologies. Addition of a closed-loop current regulating circuit enables practical application of cold cathode technology in areas where cathode noise and instability have historically been insurmountable limiting factors.
A closed-loop, cold cathode current regulator in accordance with the present invention comprises a cold cathode having an emitter and an electrode that cooperates to extract electrons from the emitter, a current limiting element that controls the flow of electrons to the emitter in response to a control signal, a current sensing element that produces an output signal that is a function of the current flowing from the emitter and a current control element that based upon the output signal of the current sensing element and a reference level produces the control signal.
In one embodiment of the invention the reference level is produced by a reference element that provides a fixed set point input to the current control element. The reference element may comprise a resistive voltage divider that derives a voltage output signal from the circuit power supply. In an additional embodiment of the invention the reference element may include a resistive voltage divider and a non-inverting gain element. In this embodiment the voltage output signal is derived from the circuit power supply, but with a rate of change that differs from that of the circuit power supply.
In an alternate embodiment of the invention the reference level is produced by a time-varying input source. The time variation may represent the transmission of analog or digital information that can be intended for aural, visual or data processing interpretation.
Additionally, the current regulator circuit of the present invention may include a circuit power supply, which may provide power to the circuit at a fixed direct-current (DC) voltage. Alternatively, the circuit power may be derived from an alternating-current (AC) source, transformer-coupled, with rectifier and filter circuitry that can allow the voltage applied to the circuit to vary in proportion to the magnitude of the AC source at the input. The regulator circuit also may include, if required for the particular cold cathode of interest, a cathode bias supply that can draw power either from the supply for the current regulating circuit or from an external source.
Additionally the present invention is embodied in a method for regulating current in a cold cathode using a closed-loop circuit. The method comprises producing a reference level based upon a set point current for a cold cathode emitter, producing a sensing output that is a function of the current flowing from the emitter, comparing the sensing output and the reference level to produce a control signal and regulating the flow of electrons to a cold cathode in response to the control signal.
As such, the present invention is capable of providing for a closed-loop regulator circuit that provides field-based, cold cathodes with markedly improved current stabilization. Additionally, further embodiments of the invention allow the emission current to be controlled remotely, without direct adjustment of the circuit. These benefits have wide spread applicability to numerous cathode devices, including but not limited to, analytical devices (e.g. scanning electron microscopes), CRT monitors and the like.